This invention generally relates to fluorescent lamps and, more particularly, to an improved fluorescent lamp of warm-white color which has both good efficacy and good color rendering properties.
Fluorescent lamps are fabricated to provide various color temperatures of emission which generally correspond to the color temperature of a complete or full radiator maintained at the specified temperature. A warm-white correlated color temperature is specified as being about 3,000.degree. K. and cool-white color temperature is specified as being about 4,100.degree. K. For illumination in the home, the public seems to prefer the warm color, possibly due to long-time familiarity with the incandescent lamp which has a warm color.
A standard warm-white fluorescent lamp which is designed to operate with a correlated color temperature of about 3,000.degree. K. is normally fabricated with a halophosphate phosphor. Such lamps have reasonably good lumen output, a representative output being 3,100 lumens for a 40 watt lamp, but the color rendering properties of such lamps are relatively poor. Fluorescent lamps which have a warm-white color and relatively good color rendering properties have been available on the market for a number of years, but the lumen output of these lamps is relatively poor and a considerable sacrifice in lumens is made in order to obtain the good color rendering properties.
In U.S. Pat. No. 3,937,998, dated Feb. 10, 1976 to Verstegen et al is disclosed a three-component phosphor blend for use in fluorescent lamps to provide both good color rendition of illuminated objects and a high light output. The components comprising this phosphor blend are very expensive and to decrease the need for these expensive components, there is disclosed in U.S. Pat. No. 4,088,923, dated May 9, 1978, to Manders an underlayer of relatively inexpensive halophosphate phosphor of the same emission color so that the expensive phosphor blend components are "worked harder", and less material is required.
In U.S. Pat. No. 3,858,082, dated Dec. 3, 1974, to Thornton is disclosed a three-component phosphor blend for use in fluorescent lamps which provide both good color rendition of illuminated objects and a high light output. One of the components comprising this blend is expensive and to decrease the amount of this expensive component which is needed, the blend can be coated as separate layers with the expensive blend component positioned next to the discharge so that it will "work harder." Such a separate layer construction is disclosed in U.S. Pat. No. 3,602,758, dated Aug. 31, 1971 to Thornton et al.
In copending Application Ser. No. 058,574, filed July 17, 1979, by J. Van Broekhoven, one of the present applicants, and R. Corth, and owned by the present assignee, now U.S. Pat. No. 4,263,530, is disclosed a fluorescent lamp which incorporates a phosphor blend of warm-white color which comprises a mixture of calcium fluoroapatite activated by antimony and manganese and yttrium oxide activated by trivalent europium. The resulting lamp has both improved output and color rendering index as well as improved color preference index as compared to a standard fluorescent lamp which incorporates a halophosphate phosphor of the same color temperature.
The internationally accepted procedure for standardizing and measuring the color rendering properties of light sources is set forth in the publication of The International Commission on Illumination, identified as Publication CIE No. 13 (E-1.3.2) 1965. More recently, a color-preference index has been proposed for rating the performance of light sources in accordance with what the normal observer considers to be the preferred coloration for familiar objects. This color preference index (CPI) is summarized in the Journal of the Illuminating Engineering Society, pages 48-52 (October 1974) article entitled "A Validation of the Color-Preference Index" by W. A. Thornton.